Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

THE MOLECULAR MECHANISMS OF MEMBRANE TRANSPORT
701
HO
OHHO
CH 2
O
HO
O
OH
O P
CH
O
O C O C
O
CH 2
O _
3 2
PI(3,5)P 2
4 1
5 6
P
PI(3)P
PI
PI(3,4)P 2
PI(4)P
PI
(B)
(D)
PI(3,4,5)P 3
P
3 2
P 4 1
5 6
P
PI(3,4)P 2
(A) (C) (E) (F)
PI(5)P
PI(4,5)P 2
Membrane-Bending Proteins Help Deform the Membrane During
Vesicle Formation
The forces generated by clathrin coat assembly alone are not sufficient to
shape and pinch off a vesicle from the membrane. Other membrane-bending
MBoC6 m13.10/13.09
and force-generating proteins participate at every stage of the process. Membrane-bending
proteins that contain crescent-shaped domains, called BAR
domains, bind to and impose their shape on the underlying membrane via electrostatic
interactions with the lipid head groups (Figure 13–12; also see Figure
10–40). Such BAR-domain proteins are thought to help AP2 nucleate clathrin-mediated
endocytosis by shaping the plasma membrane to allow a clathrin-coated
bud to form. Some of these proteins also contain amphiphilic helices that induce
membrane curvature after being inserted as wedges into the cytoplasmic leaflet
of the membrane. Other BAR-domain proteins are important in shaping the neck
of a budding vesicle, where stabilization of sharp membrane bends is essential.
Finally, the clathrin machinery nucleates the local assembly of actin filaments
that introduce tension to help pinch off and propel the forming vesicle away from
the membrane.
Cytoplasmic Proteins Regulate the Pinching-Off and Uncoating of
Coated Vesicles
As a clathrin-coated bud grows, soluble cytoplasmic proteins, including dynamin,
assemble at the neck of each bud (Figure 13–13). Dynamin contains a PI(4,5)
P 2 -binding domain, which tethers the protein to the membrane, and a GTPase
domain, which regulates the rate at which vesicles pinch off from the membrane.
Figure 13–10 Phosphatidylinositol
(PI) and phosphoinositides (PIPs).
(A, B) The structure of PI shows the
free hydroxyl groups in the inositol
sugar that can in principle be modified.
(C) Phosphorylation of one, two, or three
of the hydroxyl groups on PI by PI and
PIP kinases produces a variety of PIP
species. They are named according to
the ring position (in parentheses) and the
number of phosphate groups (subscript)
added to PI. PI(3,4)P 2 is shown. (D) Animal
cells have several PI and PIP kinases and
a similar number of PIP phosphatases,
which are localized to different organelles,
where they are regulated to catalyze the
production of particular PIPs. The red
and green arrows show the kinase and
phosphatase reactions, respectively.
(E, F) Phosphoinositide head groups
are recognized by protein domains that
discriminate between the different forms.
In this way, select groups of proteins
containing such domains are recruited
to regions of membrane in which these
phosphoinositides are present. PI(3)P and
PI(4,5)P 2 are shown. (D, modified from
M.A. de Matteis and A. Godi, Nat. Cell Biol.
6:487–492, 2004. With permission from
Macmillan Publishers Ltd.)
phagocytosis
Figure 13–11 The intracellular location of phosphoinositides. Different
types of PIPs are located in different membranes and membrane domains,
where they are often associated with specific vesicle transport events. The
membrane of secretory vesicles, for example, contains PI(4)P. When the
vesicles fuse with the plasma membrane, a PI 5-kinase that is localized
there converts the PI(4)P into PI(4,5)P 2 . The PI(4,5)P 2 , in turn, helps recruit
adaptor proteins, which initiate the formation of a clathrin-coated pit, as the
first step in clathrin-mediated endocytosis. Once the clathrin-coated vesicle
buds off from the plasma membrane, a PI(5)P phosphatase hydrolyzes
PI(4,5)P 2 , which weakens the binding of the adaptor proteins, promoting
vesicle uncoating. We discuss phagocytosis and the distinction between
regulated and constitutive exocytosis later in the chapter. (Modified from
M.A. de Matteis and A. Godi, Nat. Cell Biol. 6:487–492, 2004. With
permission from Macmillan Publishers Ltd.)
endocytosis
regulated exocytosis
constitutive exocytosis
KEY: PI(3)P PI(4)P PI(4,5)P 2 PI(3,5)P 2 PI(3,4,5)P 3